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Inﬁnity Loop bridge

10 May, 2012 | By Gordon Affleck, Kien Hoang

How Buro Happold and 10 Design used two steel parabolic arches to create a vivid landmark for the Shizimen bridge

The Shizimen bridge, also known as the Infinity Loop bridge, is the largest of the five proposed new bridges providing access to the new Zhuhai Shizimen central business district across the Shizimen canal and is scheduled to complete in 2014.

The essential challenge to architect 10 Design and structural engineer Buro Happold was to respond to the constraints and opportunities presented by the site with a suitable structural concept, form and materials, while also creating a landmark for the new urban development.

The resulting design uses conventional materials and technology to create a structure that transcends the utilitarian requirements of typical infrastructures. Here, the architect and structural engineer explain how they came to this solution.

THE ARCHITECT

We bridged the canal with a ribbon of steel, says Gordon Affleck of 10 Design

The primary structure of the bridge is formed by a single, continuous, undulating ribbon of steel. The ribbon wraps around and over the bridge deck forming sculptural arches that provide cable stay support for the road deck as it crosses the 300 metre-wide canal opening. As well as three lanes of traffic in each direction, the bridge also caters for pedestrian and cycle paths on either side.

A single pier separates the bridge into two spans, supported by primary and secondary arch forms. The primary arch crosses the road deck diagonally as it sails above the longest clear span of some 200 metres. By crossing the bridge diagonally, the arch becomes a gateway that traverses and addresses both the canal and road.

The smaller span supports the waterside edge of the road deck and pedestrian walkway as it cantilevers out over the receding shoreline, which changes across the width of the bridge as it moves from marina to canal. The minor arch also acts as a gateway for the pedestrian boardwalk as the marina park continues below the road deck, linking with the canal-side park.

The lower edge of the ribbon supports the wider pedestrian and cycle way to the north, which looks out over the Pearl River Delta towards the mainland. The walkway dips to create a stepped viewing platform midway across the canal at the supporting pier. The platform offers a rest point and views cross the Pearl River Delta towards the three tallest structures in the region, the St Regis tower on the mainland to the North, the new Shizimen tower and the Macau tower.

Both pedestrian and cycle paths are cantilevered off the concrete road deck at a lower level to provide vehicular traffic a more open view from the bridge, while at the same time creating a visual and sound barrier between pedestrian and cycles routes and road traffic.

THE STRUCTURAL ENGINEER

We used a tied-arch system to support the concrete bridge deck, writes Kien Hoang of Buro Happold

The bridge is a cable-supported structure whose main feature is a continuous steel-sculpted structural element that encircles the deck and rises above traffic level to form a pair of arches, one spanning approximately 190 metres and crossing the deck diagonally, the other spanning approximately 100 metres, parallel to an offset from one side of the deck. The arches have springing points at the banks and within the channel.

The arches support the deck, which has a curvature in plan dictated by the masterplan alignment and requirements for crossing the waterway. The main vehicular deck is a multi-cell concrete box-girder, while combined pedestrian and cycle routes are provided as steel cantilevers attached to the edges of the main deck. A separate pedestrian deck is also provided on the northern side of the bridge, descending over the mid-channel springing point to form a belvedere with panoramic views of Macau.

The deck is supported from the arches by cables attached at both edges for the main span, and at one edge for the smaller span. These cables fan out from points near the crowns of the arches and are spaced at approximately six metre-centres at deck level.

Piled foundations provide support to the structure at each arch springing point and at the abutment.

Structural behaviour

The Shizimen bridge functions as a tied arch system. The configuration of the arches reflects the distribution of forces within the structure: curved at the crown, where loads from the deck are applied through the cables, with straight legs connecting to the springing points. The cross sections of the arches taper and are smallest at the crown and largest at the base, allowing them to act as cantilevers to resist lateral forces from wind and earthquakes. The lower portion of each arch is also filled with concrete for enhanced strength and stiffness.

On the main arch, the fan arrangement of the cables minimises the out-of-balance forces arising from its skewed relationship to the deck. The connection to the deck edges also creates cross-sectional triangulation, which increases the system’s overall stiffness and enhances the arch’s stability. The smaller arch has lateral loading from the one-sided cable arrangement and is therefore canted in the opposite direction to minimise out of balance forces.

Although the deck’s primary function is to carry traffic, it also acts as a tie member which connects the bases of the arches, thus reducing outward thrust at the springing points, simplifying and diminishing the size of the foundations. The inherent curvature of the deck and arches allows them to change their geometry with variations in temperature. Movement joints are thus limited to abutment locations, where the deck is supported on sliding bearings.

Materials and Construction

The choice of material for the bridge results from the optimum balance between structural efficiency and the requirements of construction. The stiffened steel plate construction of the arches, with its inherent high strength-to-weight ratio and ability to form the complex geometry required of the cross section, also minimises self-weight, simplifying lifting requirements.

The concrete deck of simple prismatic cross section can be constructed in-situ on falsework, which means construction was able to begin prior to the excavation of the waterway simplifying access – usually one of the governing factors in the design and construction of bridges of this scale.

Durability and maintenance

Materials and construction were chosen for long-term durability and low maintenance requirements; a major concern for bridges designed to have a lifespan of 120 years, compared to the 50 or 60 years of typical buildings. This is reflected in the use of concrete for the deck, the closest major element to the water, and also in the choice of a continuous deck-form, which reduces the number of movement joints required.

Where steel is used, it is generally in the form of closed cells, minimising exposure. All steel surfaces will have high-performance paint systems and a bridge health monitoring system will facilitate long-term maintenance.

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